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[SOLVED] Ring oscillator in Verilog-A

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gag2000

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Dear all,

I'm trying to describe a ring oscillator in Verilog AMS. I've wrote several models of the delay cell, and they are all working fine in open loop.
When I try to simulate in closed loop, either all cells are in a Vin=Vout=VDD/2 steady state (OK, according to the code), or the simulator reports error (cannot compute DCOP, normal, as this is an oscillator, there is no steady state point).
I've tried without success different initialisation for "tran" and "dc".

My question is : "how to make a ring oscillator oscillate in Verilog-ams ?"

Thank you for your valuable help !
RG

- - - Updated - - -

Ooops, after three days, and few minutes after posting, I've found a solution.
In analysis->Tran->Option->SkipDC->Yes
Now, it oscillate without sim errors.
 

In analysis->Tran->Option->SkipDC->Yes
Now, it oscillate without sim errors.
I'm not using Verilog-A, thus I didn't answer to the thread. But it's essentially the same as in any SPICE based analog simulator. You need an initial condition different from steady state to start an oscillator simulation. Skipping initial DC analysis is one option, setting explicite initial conditions for a signal a second (don't know if it's possible in Verilog-A). Finally you can implement a kick-start, some disturbance injected to the ring after start. It must work with Veilog-A, too.
 

Dear FvM,
Thank you for your reply.
I had tried this by two means:
- add initial net condition in spectre, but in this case, the simulation is failing (unable to compute DCOP)
- add a signal with a coupling capacitor to start oscillating, but the oscillator keep the vdd/2 steady-state. In this case, my code is probably in fault !
 

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